In the Bioko model of human evolution, human skin evolved from neutral-colored chimpanzee skin. The aquatic apes lost their fur and evolved into naked humans, because they were mostly in the water, at least during daylight hours. Water covered most of their bodies, and hair covered the tops of their heads.
Humans with neutral-colored skin presumably left Bioko during the last 300,000 years, when there were 3 or 4 periods of land bridges. The last humans may have left as recently as 50,000 years ago. They quickly invented clothing for protection during the day and warmth at night, but there was still some skin exposed to the elements. Their skin colors evolved according to where they migrated.
Lighter skin was selected for in Europe and Asia, because it helped convert the UV of sunlight to vitamin D. Darker skin was selected for in sunny African climates, because it helped protect against too much UV, which causes skin cancer.
Here is a map of the world showing UV levels. Note that the African coast in the Gulf of Guinea is relatively light, because it is often cloudy there. Bioko itself is not shown on this map, but it would probably be one of the lightest colors, because it has its own landscape and climate. Wikipedia: Bioko has a rather debilitating climate. The so-called dry season lasts from November to March, and the rest of the year is rainy. The average annual temperature of about 77 °F (25 °C) varies little throughout the year. Afternoon temperatures reach the high 80s °F (low 30s °C) and drop to only about 70 °F (21.1 °C) at night. Most of the time the sky is cloudy and overcast.